Cast epoxy resins are routinely used within the electrical industry as a replacement for metal and porcelain in such articles as transformer bushings. Epoxy resins are high in strength, low in shrinkage, and have excellent electrical properties. Their main disadvantage for replacement use in transformer bushings is their high thermal expansion.
Metals, such as aluminum, copper and stainless steel, commonly used in electrical apparatus, have coefficients of linear thermal expansion which are much lower than epoxy resins. During thermal cycling of an electrical assembly insulated with an epoxy resin, the stresses imposed can cause cracking of the insulation, as well as separation of the metal from the applied epoxy resin, with ultimate failure of the electrical assembly.
This problem was solved by Hofmann, in U.S. Pat. Nos. 3,434,087 and 3,547,871, and Tsukui, in U.S. Pat. No. 3,658,750. They incorporated low expansion filler materials, within critical particle size ranges, into the epoxy, to form a low expansion resin insulation system. The use of certain particle size distributions allowed high filler loading while maintaining good flow properties during casting, and the high filler loading allowed a close match of the applied resin system and the metal expansion characteristics.
The fillers used in the insulating resin systems included silicon dioxide as sand, fused silica or quartz; alumina; magnesia; zirconia; calcium oxide; zirconium silicate; calcium silicate; magnesium silicate; aluminum silicate; beryllium aluminum silicate; lithium aluminum silicate; barium sulfate; calcium sulfate; barium carbonate; calcium carbonate; cobalt sulfide; cadmium sulfide; cuprous sulfide; and cupric sulfide powder, alone or in combination.
The main all-purpose filler generally used is silicon dioxide, in the form of fused silica. Fused silica has a very low coefficient of linear thermal expansion (C.L.T.E.) of about 0.5 .times. 10.sup..sup.-6 in./in./.degree. C., and a resin compatibility that allows high filler loading of the epoxy resin. The use of fused silica filler can lower the C.L.T.E. of the resin system to about 25 .times. 10.sup..sup.-6 in./in./.degree. C., so that it can be bonded successfully to a majority of metals, for example copper, having a C.L.T.E. of about 17 .times. 10.sup..sup.-6 in./in./.degree. C. The main disadvantages of fused silica are its relatively high cost, low thermal conductivity and limited supply.
It is desirable, therefore, that a new and improved filler be developed which can be easily produced, has a low cost, a high thermal conductivity, a low coefficient of thermal expansion, and a surface chemistry that will allow the filler to favorably react with and be wet by the epoxy resin. The filler must also allow excellent fluidity of the filled resinous composition at between 70 to 85 weight percent filler loading so that it is easily castable.